From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 8

substrate chain, and then leaves the substrate and binds to another one. There is no bind-
ing tendency between the enzyme and molecular sequence of the substrate [26]. Strains
that produce a large amount of extracellular deacetylase with high activity are very valu-
able in the production of chitin deacetylase and the production of chitosan by the catalytic
method.
The chitin deacetylase method could replace the hot concentrated alkali method because
it prevents serious environmental pollution, lowers energy consumption, and solves the
problem that product treated with hot concentrated alkali has uneven DD and low relative
molecular weight. The product formed by the enzyme method can be used for producing
new functional materials. Nevertheless, there are still some problems such as low yields of
deacetylase-producing strains and low enzyme activity. Moreover, natural chitins are
crystals, not a good substrate for deacetylase. Hence, many preparations still need to be
carried out before the chitin deacetylase method can be used in the industrial production
of chitosan.
The microorganism culture method is another hotspot of chitosan research, which
removes acetyls by catalyzing the substrate with enzymes produced by microorganisms.
From the 1980s, Japan and the United States began to study chitosan production by micro-
bial fermentation [27-29], followed by China from the early 1990s. Currently, the research
concentrates on breeding of the strain and optimization of the culture medium. The chito-
san formed by this method is similar to the chitosan from shelled animals in terms of DD
and relative molecular weight, while its metal ion adsorption capacity is much higher. So
the product is particularly suitable for treating heavy-metal-ion-containing wastewater.
The antibacterial ability of the food preservative made of the product is 1-2 times that of
the food preservative made of chitosan from shelled animals. It can be seen that the micro-
organism culture method has good prospects.
1.3 Control of Quality
1.3.1 Deacetylation Degree
The DD of chitosan, namely the content of free amino in chitosan chains, is a technical index
of great importance. The DD of chitosan directly relates to solubility in diluted acid, viscos-
ity, ion exchange ability, flocculability, reaction capacity with amino, and other aspects.
DD can be defined as the ratio of residues without acetyls to all residues of chitosan.
Quite a few methods can be used for measuring DD, such as alkalimetry (acid-base
titration [30-33], electrolytic titration [34-35], and hydrobromide titration [36]), infrared
spectroscopy [37-41], refractive index [42], colloid titration [43,44], thermal analysis [45],
gas chromatography [46], ultimate analysis [46], ultraviolet (UV) spectrometry [44,45], and
trinitrophenol spectrophotometry [37]. The most common method is acid-base titration,
followed by infrared spectroscopy and electrolytic titration.
Acid-base titration is the simplest method with good repetitiveness for measuring the
content of free amino in chitosan, and does not require special instruments. This method
is particularly suitable for monitoring weight during production. The mechanism is that
the alkali free amino in chitosan can be protonated by acid quantification to form a chito-
san colloid solution, then the dissociative hydrogen ions can be titrated by alkali, and acid
combined by free amino can be figured out by the difference of acid for dissolving chito-
san and alkali for titrating.